1) Field of the Invention
The present invention relates to a thermotropic aromatic polyester. More particularly, the present invention relates to a thermotropic wholly aromatic polyester which can be melt-processed at a relatively low temperature of, for example, about 400.degree. C. or less, and exhibits a liquid crystalline property when melted.
2) Description of the Related Arts
Among the various types of engineering plastic resins recently developed, liquid crystalline polymers having an optical anisotropic property are attracting particular attention.
The known liquid crystalline polymers are p-hydroxybenzoic acid homopolymer and terephthalic acid-hydroquinone polymers, and these known polymers have high melting points of 610.degree. C. and 596.degree. C., respectively, and therefore, it is difficult to melt-process these polymers without a decomposition thereof, as mentioned in "Advances in Polymer Science", 60/60, 61 (1984).
Japanese Examined Patent Publication No. 47-47870 discloses a liquid crystalline polymer consisting of a p-hydroxybenzoic acid-terephthalic acid and hydroquinone copolymer, but this polymer has a high melting point of 500.degree. C. or more, and thus has an unsatisfactory melt-processability.
"Brit. Polymer Journal", 132 (1980) discloses various attempts to provide liquid crystalline polymers having a relatively low melting point, and "Makromol. Chem.", 189, 2023 (1988) and "Makromol. Chem. Makromol Symp." 26, 47 (1989) disclose polyesters containing, as a comonomeric component, 2,2'-dimethylbiphenyl-4,4'-dicarboxylic acid. Also, "Journal of Polymer Sci., Part C, Polymer Letters, 25, 11 (1987), and "Makromolecules, 20, 2374 (1987) disclose polyesters containing, as a monomeric component, 2,2'-bis(trifluoromethyl)-biphenyl-4,4'-dicarboxylic acid.
Nevertheless, it was found that, when 2,2'-di-substituted biphenyl-4,4-dicarboxylic acids are employed as a monomeric component, the resultant polymer is disadvantageous in that the coplanality of the polymer is hindered by the substituents of this monomeric component, and thus the crystallinity of the polymer is lowered, and in general, polymers having a lowered crystallinity exhibit an unsatisfactory mechanical strength.